Thin flat meters typically on the order of one-half inch thick in the direction of the axis of the meter movement are known in the art. Such shallow meters are frequently referred to as "compact". A typical mechanism for such meters is disclosed in U.S. Pat. No. 3,621,393, issued Nov. 16, 1971.
In the mechanism of the above-mentioned patent, cements and epoxies are used to secure the parts of the rotor or moving element together, and cement is also used to secure the permanent magnet to a first magnetically permeable plate of the mechanism. A second magnetically permeable plate parallel to the first plate, is magnetically connected to the first plate by spacers and screws, to provide an arcuate gap to accommodate the moving coil for movement along the first plate and magnet.
While the construction disclosed in this aforementioned patent provides a meter of good accuracy which can be assembled on a mass production basis, the meter is somewhat expensive, because of the number of parts which must be separately manufactured and aligned during assembly.
A desirable feature of such meters is an air gap of arcuate section in a plane perpendicular to the pivot axis and generally parallel with or including the plane of swing of the pointer. In order to obtain consistency or repeatability from successively assembled meters, it is necessary that the air gap characteristics be uniform from meter to meter. It is however preferred that the flux in the air gap be uniform throughout the air gap so the meter movement has linear characteristics, i.e. deflects the same angular distance for each increment of current which energizes its winding. In a mechanism of the type disclosed in the aforementioned patent, cement was used to secure the magnet to one of the magnetically permeable legs. While the cement technique was satisfactory, cementing is expensive, and the deflection characteristics of the instrument sometimes varied from meter to meter. In addition, consistent magnetic coupling between the magnetically permeable plates was difficult to obtain because of the separate studs which magnetically coupled these plates together.
In the meter of the aforementioned patent the moving coil extends around both a permeable magnetic plate and the magnet cemented to the plate. Such an arrangement requires a coil of substantial axial height with substantial coil wire to provide the number of turns needed to cause deflection where the sensitivity of the meter is in the micro-amp range.
Correspondingly, even though many accurate meters of the construction of the aforementioned patent were successfully produced, and even though excellent accuracy was obtained with many of these meters, some construction difficulties, as mentioned above, were encountered.